Interpretive Summary: Ground-level ozone is a damaging air pollutant that currently decreases crop productivity. The effects of ozone are dependent upon other environmental conditions, including the nitrogen status of the crop. In this study, we examined tobacco grown at ambient and elevated ozone concentrations under two nitrogen levels to determine how ozone impacts tobacco growth and physiology under limiting and sufficient nitrogen. Limiting nitrogen availability reduced photosynthesis and growth of tobacco, and exposure to elevated ozone exacerbated those reductions. In addition, ozone accelerated leaf senescence in both limiting and sufficient nitrogen conditions. These results suggest that crop productivity in areas with low fertility will be significantly impacted by ozone, and that even in areas with high fertility, ozone will accelerate senescence of crops. This work is important for producers and crop modelers who aim to maximize production in an ozone polluted atmosphere.

Technical Abstract:
Elevated ozone (O3) and limiting soil nitrogen (N) availability both negatively affect crop performance. However, little is known about how the combination of elevated O3 and limiting N affect crop growth and metabolism. In this study, we grew tobacco (Nicotiana sylvestris) in ambient and elevated O3 at two N levels (limiting and sufficient). Results at the whole plant, leaf and cellular level showed that primary metabolism was reduced by growth in limiting N, and that reduction was exacerbated by exposure to elevated O3. Limiting N availability reduced the rates of photosynthetic CO2 uptake by 40.8% in ambient O3- exposed plants, and by 58.6% in elevated O3-exposed plants, compared to plants grown with sufficient N. Reductions in photosynthesis compounded to cause large differences in leaf and whole plant parameters such as leaf number, leaf area and leaf and root biomass. N uptake and allocation was also affected by growth in limiting N and elevated O3, and there was an O3-induced compensatory response that resulted in increased N recycling from senescing leaves. In addition, transcript-based markers were used to track the progress through senescence, and indicated that limiting N and elevated O3, separately and in combination, caused an acceleration of senescence. The results also showed that at the molecular level, senescence was accelerated by exposure to O3, even in plants grown in sufficient N. These results suggest that reductions in crop productivity in growing regions with poor soil fertility will be reduced furtherexacerbated by elevated O3. Therefore, N management may not be an effective strategy to mitigate all of the negative effects of elevated O3.